Structured surfaces comprising a plurality of micro-scale and/or nano-scale surface elements are used in a variety of technical fields and applications including, for example, optical applications and so-called self-cleaning surfaces.
Natural surfaces such as plant leaves or animal furs may exhibit self-cleaning surfaces employing the so-called Lotus effect to provide for water repellence. The term Lotus effect has been coined by botanist Wilhelm Barthlott who has conducted extensive investigations of this phenomenon since about 30 years (Barthlott, Wilhelm; Ehler, N. (1977). “Raster-Elektronenmikroskopie der Epidermis-Oberflächen von Spermatophyten”. Tropische and subtropische Pflanzenwelt (Akad. Wiss. Lit. Mainz) 19: 110). The Lotus effect is based on the presence of micro- and/or sub-micrometer (i.e. nanometer)-sized mechanical structures on the surface which by virtue of their shape, size, and distance prevent water droplets from spreading out and from penetrating through the surface into the substance. Typical rain drops may be about 5 mm in diameter, and a surface providing a water-repellent effect exhibits a structure having surface elements which are much smaller than the size of a water droplet and typically comprise surface elements of a micrometer and/or nanometer scale. This relation between the typical sizes of a water droplet and the surface elements of a water-repellent surface tends to result in a decrease of the contact area between the water droplet and the surface. Thus wetting of the surface is minimized, and so is the holding power of the droplet to the surface. As a result, the drop tends to roll of the surface when that surface is only slightly tilted, taking loose dirt particles with it as it rolls off. This is the reason why such surfaces are often referred to as self-cleaning.
It is known that the water repellent effect of hydrophobic materials can be enhanced by employing such materials in an appropriately structured surface.
WO 96/04123 and U.S. Pat. No. 6,641,767 disclose water-repellent micro-structured surfaces comprising a low-surface-energy material.
WO 96/04,123 discloses self-cleaning surfaces comprising hydrophobic polymer materials. The surfaces exhibit artificial surface elements having a height of between 5 μm (micrometers) to 100 μm and a distance of between 5 μm to 200 μm between the elements.
U.S. Pat. No. 6,641,767 discloses a method of forming a replicate of a structured surface. The tool surfaces from which the surfaces are obtained by replication comprise a plurality of pyramidal shaped surface elements exhibiting a continuous distribution of their sizes ranging from less than 50 nm (nanometers) to greater than 1 μm. Such surface elements are obtained in U.S. Pat. No. 6,641,767 by vapour-coating deposition.
Micro- and/or nano-structured surfaces disclosed in the prior art tend to have an insufficient mechanical stability. WO 96/04,123, for example, discloses that the surfaces specified therein may not be exposed to high mechanical stresses during cleaning with rinsing water as this may polish and even out the surface elements so that the surfaces loose their self-cleaning capability. WO 96/04,123 also discusses an insufficient mechanical stability of micro- and/or nano-structured surfaces.
European patent EP 0,933,388 discloses a surface that has nano- and/or microscale surface elements having an average height of 50 nm to 10 μm and an average distance of 50 nm to 4 μm. The micro- and nano-scale surface elements are arranged as a “superstructure” on more coarse microscale surface elements having an average height of 10 μm to 1 mm (millimeter) and average spacing of 10 μm up to 1 mm. Structured surfaces of this type are generally also referred to as “structure-on-structure” (Sons) surfaces. EP 0,933,388 discloses that the SonS structure provides for an enhanced mechanical resistance when compared to non-SonS surfaces comprising micro- and nano-size surface elements. It is speculated that this results from a mechanical protection of the nano- and or micro-scale surface elements in the indentations or valleys of the micro-scale superstructure.
However, the problem how to provide an optionally hydrophobic durable structured surface comprising a plurality of micro-scale and/or nano-scale surface elements that exhibits a favourable mechanical stability and a high scratch resistance remains unsolved.
It was therefore an object of the present invention to provide a structured surface comprising a plurality of micro-scale and/or nano-scale surface elements that exhibits favourable mechanical properties and, in particular, a high scratch resistance. It is another object of the present invention to provide a hydrophobic structured surface comprising a plurality of micro-scale and/or nano-scale surface elements that maintains its hydrophobic character when subjected to mechanical stress. It is, in particular, an object of the present invention to provide a scratch-resistant structured surface including a plurality of both micro- and nano-scale surface elements having a high static contact angle against water of at least 130°.